be going to / will
用于条件句时， be going to 表将来
will 表意愿
If you are going to make a journey, you'd better get ready for it as soon as possible.
Now if you will take off your clothes, we will fit the new clothes on you in front of the mirror.
大学英语四级语法知识：be to和be going to
be to和be going to
be to 表示客观安排或受人指示而做某事。
be going to 表示主观的打算或计划。
I am to play football tomorrow afternoon. (客观安排)
I'm going to play football tomorrow afternoon. (主观安排)
From:https://www.zgghmh.com/CET-4/20001539.asp